Flip chip interconnection having narrow interconnection sites on the substrate

ABSTRACT

A flip chip interconnect of a die on a substrate is made by mating the interconnect bump onto a narrow interconnect pad on a lead or trace, rather than onto a capture pad. The width of the narrow interconnect pad is less than a base diameter of bumps on the die to be attached. Also, a flip chip package includes a die having solder bumps attached to interconnect pads in an active surface, and a substrate having narrow interconnect pads on electrically conductive traces in a die attach surface, in which the bumps are mated onto the narrow pads on the traces.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional Application No.60/665,208, filed Mar. 25, 2005, titled “Flip chip interconnectionhaving narrow interconnection sites on the substrate”, which is herebyincorporated herein by reference.

This application is related to U.S. application Ser. No. 10/985,654,filed Nov. 10, 2004, titled “Bump-on-lead flip chip interconnection”.

BACKGROUND

This invention relates to semiconductor packaging and, particularly, toflip chip interconnection.

Flip chip packages include a semiconductor die mounted onto a packagesubstrate with the active side of the die facing the substrate. Thesubstrate is made up of a dielectric layer and at least one metal layer,patterned to provide substrate circuitry, which includes among otherfeatures traces (“leads”) leading to interconnect pads. The metal layermay be patterned by, for example, a mask-and etch process.Conventionally, interconnection of the circuitry in the die withcircuitry in the substrate is made by way of bumps which are attached toan array of interconnect pads on the die, and bonded to a corresponding(complementary) array of interconnect pads (often referred to as“capture pads”) on the substrate. The capture pads are typically muchwider than the leads, and can be as wide as, for example, about 2 to 4times the nominal or design width of the leads.

The areal density of electronic features on integrated circuits hasincreased enormously, and chips having a greater density of circuitfeatures also may have a greater density of sites (“die pads”) forinterconnection with the circuitry on a package substrate.

The package is connected to underlying circuitry, such as a printedcircuit board (e.g., a “motherboard”), in the device in which thepackage is employed, by way of second level interconnects (e.g., pins,secondary interconnect solder balls) between the package and theunderlying circuit. The second level interconnects have a greater pitchthan the flip chip interconnects, and so the routing on the substrateconventionally “fans out”. Significant technological advances inpatterning the metal layer on the substrate have enabled construction offine lines and spaces; but in the conventional arrangement space betweenadjacent pads limits the number of traces than can escape from the moreinward capture pads in the array, and the fan out routing between thecapture pads beneath the die and the external pins of the package isconventionally formed on multiple metal layers within the packagesubstrate. For a complex interconnect array, substrates having multiplelayers may be required to achieve routing between the die pads and thesecond level interconnects on the package.

Multiple layer substrates are expensive, and in conventional flip chipconstructs the substrate alone typically accounts for more than half thepackage cost (about 60% in some typical instances). The high cost ofmultilayer substrates has been a factor in limiting proliferation offlip chip technology in mainstream products.

In conventional flip chip constructs the escape routing patterntypically introduces additional electrical parasitics, because therouting includes short runs of unshielded wiring and vias between wiringlayers in the signal transmission path. Electrical parasitics cansignificantly limit package performance.

SUMMARY

According to the invention flip chip interconnect is accomplished byconnecting the interconnect bump directly onto a narrow interconnectionpad, or narrow pad, rather than onto a conventional capture pad. Thewidth of the narrow pad according to the invention is selected accordingto the base diameter of the interconnect bump that is to be connectedonto the narrow pad. Particularly, the width of the narrow pad is less(such as in a range about 20% to about 80%) than the base diameter ofthe interconnect bump. The invention provides more efficient routing oftraces on the substrate. Particularly, the signal routing can be formedentirely in a single metal layer of the substrate. This reduces thenumber of layers in the substrate, and forming the signal traces in asingle layer also permits relaxation of some of the via, line and spacedesign rules that the substrate must meet. This simplification of thesubstrate greatly reduces the overall cost of the flip chip package. Thebump-on-narrow-pad architecture also helps eliminate such features asvias and “stubs” from the substrate design, and enables a microstripcontrolled impedance electrical environment for signal transmission,thereby greatly improving performance.

In one general aspect the invention features a flip chip interconnectionhaving solder bumps attached to interconnect pads on a die and matedonto corresponding narrow interconnection pads on a substrate.

In another general aspect the invention features a flip chip packageincluding a die having solder bumps attached to interconnect pads in anactive surface, and a substrate having narrow interconnection pads in adie attach surface, in which the bumps are mated onto the narrow pads.

In some embodiments the bump-on-narrow-pad interconnection is formedaccording to methods of the invention without use of a solder mask toconfine the molten solder during a re-melt stage in the process.Avoiding the need for a solder mask allows for finer interconnectiongeometry.

In some embodiments the substrate is further provided with a solder maskhaving openings over the narrow interconnection pads. In someembodiments the substrate is further provided with solder paste on thenarrow interconnection pads.

In another general aspect the invention features a method for formingflip chip interconnection, by providing a substrate having narrowinterconnection pads formed in a die attach surface and a die havingbumps attached to interconnect pads in an active surface; supporting thesubstrate and the die; dispensing a quantity of a curable adhesive onthe substrate (covering at least the narrow interconnection pads) or onthe active side of the die (covering at least the bumps); positioningthe die with the active side of the die toward the die attach surface ofthe substrate, and aligning the die and substrate and moving one towardthe other so that the bumps contact the corresponding narrowinterconnection pads on the substrate; applying a force to press thebumps onto the mating narrow pads, sufficient to displace the adhesivefrom between the bump and the mating narrow pad; at least partiallycuring the adhesive; melting and then re-solidifying the solder, forminga metallurgical interconnection between the bump and the narrow pad.

In another general aspect the invention features a method for formingflip chip interconnection, by providing a substrate having narrowinterconnection pads formed in a die attach surface and having a soldermask having openings over the narrow pads, and a die having bumpsattached to interconnect pads in an active surface; supporting thesubstrate and the die; positioning the die with the active side of thedie toward the die attach surface of the substrate, and aligning the dieand substrate and moving one toward the other so that the bumps contactthe corresponding narrow pads on the substrate; melting and thenre-solidifying to form the interconnection between the bump and thenarrow pad.

In some embodiments the solder bump includes a collapsible solderportion, and the melt and solidifying step melts the bump to form theinterconnection on the narrow pad. In some embodiments the substrate isprovided with a solder paste on the narrow pads, and the step of movingthe die and the substrate toward one another effects a contact betweenthe bumps and the solder on the narrow pads, and the melt andsolidifying step melts the solder on the narrow pad to form theinterconnection.

In another general aspect the invention features a method for formingflip chip interconnection, by providing a substrate having narrowinterconnection pads formed in a die attach surface and having a soldermask having openings over the narrow pads and having solder paste on thenarrow pads, and a die having bumps attached to interconnect pads in anactive surface; supporting the substrate and the die; positioning thedie with the active side of the die toward the die attach surface of thesubstrate, and aligning the die and substrate and moving one toward theother so that the bumps contact the solder paste on the correspondingnarrow pads on the substrate; melting and then re-solidifying the solderpaste, forming a metallurgical interconnection between the bump and thenarrow pad.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic sketch of a portion of a conventionalbump-on-capture pad flip chip interconnection, in a sectional viewparallel to the plane of the package substrate surface, as indicated bythe arrows 1-1′ in FIG. 2.

FIG. 2 is a diagrammatic sketch showing a portion of a conventionalbump-on-capture pad flip chip interconnection, in a sectional viewperpendicular to the plane of the package substrate surface, asindicated by the arrows 2-2′ in FIG. 1.

FIG. 3 is a diagrammatic sketch showing a portion of anotherconventional bump-on-capture pad flip chip interconnection, in asectional view perpendicular to the plane of the package substratesurface.

FIG. 4 is a diagrammatic sketch of a portion of an embodiment of abump-on-narrow-pad flip chip interconnection according to an embodimentof the invention, in a sectional view parallel to the plane of thepackage substrate surface.

FIG. 5 is a diagrammatic sketch showing a portion of an embodiment of abump-on-narrow-pad flip chip interconnection according to an embodimentof the invention as in FIG. 4, in a sectional view perpendicular to theplane of the package substrate surface, as indicated by the arrows 5-5′in FIG. 4.

FIG. 6 is a diagrammatic sketch of a portion of another embodiment of abump-on-narrow-pad flip chip interconnection according to an embodimentof the invention, in a sectional view parallel to the plane of thepackage substrate surface.

FIG. 7 is a diagrammatic sketch showing a portion of an embodiment of abump-on-narrow-pad flip chip interconnection according to an embodimentof the invention as in FIG. 6, in a sectional view perpendicular to theplane of the package substrate surface, as indicated by the arrows 7-7′in FIG. 6.

FIGS. 8 and 9 are diagrammatic sketches, each of a portion of anotherembodiment of a bump-on-narrow-pad flip chip interconnection accordingto an embodiment of the invention, in a sectional view parallel to theplane of the package substrate surface.

FIGS. 10A-10C are diagrammatic sketches in a sectional view illustratingsteps in a process for making a flip chip interconnection according toan embodiment of the invention.

FIGS. 11A-11D are diagrammatic sketches in a sectional view illustratingsteps in a process for making a flip chip interconnection according toan embodiment of the invention.

FIG. 12 is a diagrammatic sketch showing a force and temperatureschedule for a process for making a flip chip interconnection accordingto an embodiment of the invention.

FIG. 13 is a diagrammatic sketch in a sectional view showing abump-on-narrow-pad flip chip interconnection according to an embodimentof the invention.

FIGS. 14A-14E are diagrammatic sketches in plan view illustratingvarious interconnect pad shapes according to embodiments of theinvention.

FIGS. 15A-15C are diagrammatic sketches in plan view illustratingvarious interconnect pad configurations according to embodiments of theinvention.

FIGS. 16A and 16B are diagrammatic sketches in plan view illustratingsolder mask openings according to embodiments of the invention.

FIG. 17 is a diagrammatic sketch in plan view illustrating details ofvarious interconnect pad configurations in relation to a solder maskopening according to embodiments of the invention.

FIG. 18 is a diagrammatic sketch in plan view illustrating details ofvarious solder mask configurations in relation to an interconnect padaccording to embodiments of the invention.

DETAILED DESCRIPTION

The invention will now be described in further detail by reference tothe drawings, which illustrate alternative embodiments of the invention.The drawings are diagrammatic, showing features of the invention andtheir relation to other features and structures, and are not made toscale. For improved clarity of presentation, in the FIGs. illustratingembodiments of the invention, elements corresponding to elements shownin other drawings are not all particularly renumbered, although they areall readily identifiable in all the FIGs.

The conventional flip chip interconnection is made by using a meltingprocess to join the bumps (conventionally, solder bumps) onto the matingsurfaces of the corresponding capture pads and, accordingly, this isknown as a “bump-on-capture pad” (“BOC”) interconnect. Two features areevident in the BOC design: first, a comparatively large capture pad isrequired to mate with the bump on the die; second, an insulatingmaterial, typically known as a “solder mask” is required to confine theflow of solder during the interconnection process. The solder maskopening may define the contour of the melted solder at the capture pad(“solder mask defined”), or the solder contour may not be defined by themask opening (“non-solder mask defined”); in the latter case—as in theexample of FIG. 1, described in more detail below—the solder maskopening may be significantly larger than the capture pad. The techniquesfor defining solder mask openings have wide tolerance ranges.Consequently, for a solder mask defined bump configuration, the capturepad must be large (typically considerably larger than the design sizefor the mask opening), to ensure that the mask opening will be locatedon the mating surface of the pad; and for a non-solder mask defined bumpconfiguration, the solder mask opening must be larger than the capturepad. The width of capture pads (or diameter, for circular pads) istypically about the same as the ball (or bump) diameter, and can be asmuch as two to four times wider than the trace width. This results inconsiderable loss of routing space on the top substrate layer. Inparticular, for example, the “escape routing pitch” is much bigger thanthe finest trace pitch that the substrate technology can offer. Thismeans that a significant number of pads must be routed on lowersubstrate layers by means of short stubs and vias, often beneath thefootprint of the die, emanating from the pads in question.

FIGS. 1 and 2 show portions 10, 20 of a conventional flip chip package,in diagrammatic sectional views; the partial sectional view in FIG. 1 istaken in a plane parallel to the package substrate surface, along theline 1-1′ in FIG. 2; and the partial sectional view in FIG. 2 is takenin a plane perpendicular to the package substrate surface, along theline 2-2′ in FIG. 1. Certain features are shown as if transparent, butmany of the features in FIG. 1 are shown at least partly obscured byoverlying features. Referring now to both FIG. 1 and FIG. 2, a dieattach surface of the package substrate includes a metal layer formed ona dielectric layer 12. The metal layer is patterned to form leads 13 andcapture pads 14. An insulating layer 16 , typically termed a “soldermask”, covers the die attach surface of the substrate; the solder maskis usually constructed of a photodefinable material, and is patterned byconventional photoresist patterning techniques to leave the matingsurfaces of the capture pads 14 exposed. Interconnect bumps 15 attachedto pads on the active side of the die 18 are joined to the matingsurfaces of corresponding capture pads 14 on the substrate to formappropriate electrical interconnection between the circuitry on the dieand the leads on the substrate. After the reflowed solder is cooled toestablish the electrical connection, an underfill material 17 isintroduced into the space between the die 18 and the substrate 12 ,mechanically stabilizing the interconnects and protecting the featuresbetween the die and the substrate.

As FIG. 1 shows by way of example, signal escape traces in the uppermetal layer of the substrate (leads 13), lead from their respectivecapture pads 14 across the die edge location, indicated by the brokenline 11, and away from the die footprint. In a typical example thesignal traces may have an escape pitch P_(E) about 112 um. A 30 um/30 umdesign rule is typical for the traces themselves in a configuration asshown in FIG. 1; that is, the traces are nominally 30 um wide, and theycan be spaced as close together as 30 um. The capture pads are typicallythree times greater than the trace width and, accordingly in thisexample the capture pads have a width (or diameter, as they are roughlycircular in this example) nominally 90 um. And, in this example, theopenings in the solder mask are larger than the pads, having a nominalwidth (diameter) of 135 um.

FIGS. 1 and 2 show a non-solder mask defined solder contour. As thefusible material of the bumps on the die melt, the molten solder tendsto “wet” the metal of the leads and capture pads, and the solder tendsto “run out” over any contiguous metal surfaces that are not masked. Thesolder tends to flow along the contiguous lead 13, and here the solderflow is limited by the solder mask, for example at 19 in FIG. 1. Anon-solder mask defined solder contour at the pad is apparent in FIG. 2,in which the material of the bumps 15 is shown as having flowed, 29,over the sides of the capture pads 14 and down to the surface of thedielectric layer of the substrate 12. This is referred to as anon-solder mask defined contour because the solder mask does not limitthe flow of solder over the surface and down over the sides of thecapture pads, and—unless there is a substantial excess of solder at thepad—the flow of solder is limited by the fact that the dielectricsurface of the substrate is typically not wettable by the molten solder.A lower limit on the density of the capture pads in a conventionalarrangement, as in FIG. 1, is determined by, among other factors, limitson the capacity of the mask forming technology to make reliable narrowmask structures, and the need to provide mask structures betweenadjacent mask openings. A lower limit on the escape density isadditionally determined by, among other factors, the need for escapelines from more centrally located capture pads to be routed between moreperipherally located capture pads.

FIG. 3 shows a conventional solder mask defined solder contour, in asectional view similar to that in FIG. 2. A die 38 is shown affixed byway of bumps 35 onto the mating surfaces of capture pads 34 formed alongwith traces (leads 33) by patterning a metal layer on the die attachside of a dielectric layer of the substrate 32 . After the reflowedsolder is cooled to establish the electrical connection, an underfillmaterial 37 is introduced into the space between the die 38 and thesubstrate 32 , mechanically stabilizing the interconnects and protectingthe features between the die and the substrate. Here the capture pads 34are wider than in the example of FIGS. 1 and 2, and the solder maskopenings are smaller than the capture pads, so that the solder maskmaterial covers the sides and part of the mating surface of each capturepad, as shown at 39, as well as the leads 33. When the bumps 35 arebrought into contact with the mating surfaces of the respective capturepads 34, and then melted, the solder mask material 39 restricts the flowof the molten solder, so that the shapes of the solder contours aredefined by the shapes and dimensions of the mask openings over thecapture pads 34. In a typical example of a conventional solder maskdefined bump-on-capture pad interconnection, the capture pad has adiameter about 140 um, and the solder mask opening has a diameter about90 um, and the routing traces are about 25-30 um wide. The diameter ofthe mating surface for attachment of the bump to the die pad (not shownin FIG. 2 or 3), that is, the place of interface between the bump andthe die pad, is defined by the solder mask opening as having a diameterabout 90 um, in this example.

FIGS. 4 and 6 each show a portion of a bump-on-narrow-pad (“BONP”) flipchip interconnection according to an embodiment of the invention, in adiagrammatic partial sectional view taken in a plane parallel to thesubstrate surface, along the lines 4-4′ and 6-6′ in FIGS. 5 and 7,respectively. Certain features are shown as if transparent. According tothe invention the interconnection is achieved by mating the bumps ontorespective narrow interconnection pads on the substrate and,accordingly, this is referred to herein as a “bump-on-narrow-pad”(“BONP”) interconnect. Solder mask materials typically cannot beresolved at such fine geometries and, according to these embodiments ofthe invention, no solder mask is used. Instead the function of confiningmolten solder flow is accomplished without a solder mask in the courseof the assembly process (as described below). FIG. 5 shows a partialsectional view of a package as in FIG. 4, taken in a plane perpendicularto the plane of the package substrate surface, along the line 5-5′ inFIG. 4; and FIG. 7 shows a partial sectional view of a package as inFIG. 6, taken in a plane perpendicular to the plane of the packagesubstrate surface, along the line 7-7′ in FIG. 6.

Escape routing patterns for bump-on-narrow-pad (“BONP”) substratesaccording to the invention are shown by way of example in FIGS. 4 and 6:in FIG. 4, arranged for a die on which the die attach pads for theinterconnect balls are in a row near the die perimeter, the bumps 45 aremated onto corresponding narrow interconnection pads on the escapetraces 43 in a row near the edge of the die footprint, indicated by thebroken line 41; in FIG. 6, arranged for a die on which the die attachpads are in an array of parallel rows near the die perimeter, the bumps65 are mated onto corresponding narrow interconnection pads on theescape traces 63 in a complementary array near the edge of the diefootprint, indicated by the broken line 61.

As FIGS. 4 and 6 illustrate, the routing density achievable usingbump-on-narrow-pad interconnect according to the invention can equal thefinest trace pitch offered by the substrate technology. In the specificcase illustrated, this constitutes a routing density which isapproximately 90% higher than is achieved in a conventionalbump-on-capture pad arrangement. In the perimeter row embodiments ofBONP (e.g., FIG. 4), the bumps are placed at a fine pitch, which canequal the finest trace pitch of the substrate. This arrangement poses achallenge for the assembly process, because the bumping and bondingpitch must be very fine. In the perimeter array version of BONP (e.g.,FIG. 6), the bumps are arranged on an area array, providing greaterspace for a larger bumping and bonding pitch, and relieving thetechnological challenges for the assembly process. Even in the arrayembodiments, the routing traces on the substrate are at the sameeffective pitch as in the perimeter row arrangement, and an arrangementas in FIG. 6 relieves the burden of fine pitch bumping and bondingwithout sacrificing the fine escape routing pitch advantage.

Referring particularly now to FIGS. 4 and 5, leads 43 and narrowinterconnection pads 46 are formed by patterning a metal layer on a dieattach surface of a substrate dielectric layer 42 . The narrow pads 46are formed as a widening of the traces 43 at the interconnection sites.The “width” of an interconnection pad (W_(b) in FIG. 5) is the nominalor design dimension across the widened part of the trace at theinterconnection site. According to the invention, the width of thenarrow interconnection pad on a substrate is established according tothe bump base width (“base diameter”) of the bumps on the die that is tobe connected to the substrate. The “bump base width” (W_(p) in FIG. 5)is the nominal or design diameter of the generally round (approximatelycircular) contact interface between the bump 45 and the die pad 49. (Asmay be appreciated, the diameter of the bump, taken in a plane parallelto the bump-pad interface, may be greater than the bump base width, asillustrated diagrammatically in FIGS. 2, 3, 5 and 7, for example.)Particularly according to the invention, the interconnection pad widthW_(b) is smaller than the bump base width W_(p), and the narrowinterconnection pad width may be as small as 20% of the bump base width.In many embodiments the narrow pad width is in a range about 20% toabout 80% of the bump base width. In some embodiments the narrowinterconnection pad width is less than the bump base width and greaterthan about 25% of the bump base width. In some embodiments the narrowpad width is less than about 60% of the bump base width.

According to the invention, electrical interconnection of the die 48 ismade by joining the bumps 45 on the die onto the narrow interconnectionpads 46 on the leads 43. The conventional comparatively wide capture padis unnecessary according to the invention and, in embodiments as inFIGS. 4 and 5, no solder mask is required; the process is described indetail below.

Conventional capture pads typically are about the same width (diameter)as the bumps, and are typically two to four times as wide as the traceor lead width. As will be appreciated, some variation in the width ofleads is expected. As used herein, a narrow interconnection pad has anominal or design width at least about 120% of the nominal or tracedesign rule width, and bump-on-narrow-lead interconnection according tothe invention includes bumps connected to widened parts of traces thatare greater than about 120% of the nominal or trace design rule width,and less than the bump base diameter. An interconnection site that has awidth less than about 120% does not constitute a narrow interconnectpad, and interconnection made by connecting bumps onto portions of leadsthat are less than about 120% of the nominal or trace design rule widthis referred to as a “bump-on-lead” interconnection.

Similarly, referring to FIGS. 6 and 7, leads 63 and narrowinterconnection pads 66 are formed by patterning a metal layer on a dieattach surface of a substrate dielectric layer 62 . The signal escapetraces lead across the die edge location, indicated by the broken line61, and away from the die footprint. The narrow pads 66 are formed as awidening of the traces 63 at the interconnection sites. The “width” ofan interconnection pad (W_(b) in FIG. 7) is the nominal or designdimension across the widened part of the trace at the interconnectionsite. In this example, as in the example shown in FIGS. 4 and 5,according to the invention, the width of the narrow interconnection padon a substrate is established according to the bump base width of thebumps on the die that is to be connected to the substrate. The “bumpbase width” (W_(p) in FIG. 7) is the nominal or design diameter of thegenerally round (approximately circular) contact interface between thebump 65 and the die pad 69. Particularly according to the invention, theinterconnection pad width W_(b) is smaller than the bump base widthW_(p), and the narrow interconnection pad width may be as small as 20%of the bump base width. In many embodiments the narrow pad width is in arange about 20% to about 80% of the bump base width. In some embodimentsthe narrow interconnection pad width is less than the bump base widthand greater than about 25% of the bump base width. In some embodimentsthe narrow pad width is less than about 60% of the bump base width.

According to the invention, electrical interconnection of the die 68 ismade by joining the bumps 65 on the die narrow interconnection pads 66on the leads 63. Certain of the escape traces, e.g. 66, leading acrossthe die edge location from interconnect sites in rows toward theinterior of the die footprint, pass between the bumps 65 on moreperipheral rows of interconnect sites. No capture pads are requiredaccording to the invention and, in embodiments as in FIGS. 6 and 7, nosolder mask is required; the process is described in detail below.

According to the invention, as the techniques for forming the tracesimproves, it is possible to reliably form traces having nominal ordesign rule widths less than about 25 um. The reduced trace widths canprovide for increased routing density. However, the mechanicalreliability of a “bump-on-lead” flip chip interconnect on leads lessthan about 25 um may be unsatisfactory, because the dimensions of theinterface between the bump and the lead are small, and may not providesufficient bonding strength to provide a good electricalinterconnection. The invention provides for reliable mechanicalconnection (and good electrical interconnection) by forming a narrowinterconnect pad by widening the lead to an extent dimensionally relatedto the bump base diameter, and limited to less than the bump basediameter.

The narrow interconnect pad according to the invention may be shaped inany of a variety of ways. Some such shapes may be more readilymanufacturable, and some may provide other process advantages. Forexample, the narrow pad may be generally rectangular, either square orelongated, as shown for example in FIGS. 14A and 14B; or, it may begenerally round, either circular or elliptical, as shown for example inFIGS. 14C and 14D. Other shapes may be employed; one particularly usefulshape is shown by way of example in FIG. 14E, having semicircularportions separated lengthwise the lead or trace by a square orrectangular portion. Also, the narrow pad can be formed as a symmetricalor an asymmetrical widening in the lead or trace, as shown in FIGS. 15Aand 15B (showing a generally rectangular pad as an example). Also, thenarrow pad need not be situated at, or near, the end of the lead ortrace, but may be formed at any point where interconnection isspecified, as illustrated in FIG. 15C (showing a generally rectangularpad as an example). Forming the pad longer than wide increases thewettable mating surface of the narrow pad (planar surface plus theexposed parts of the sides), and can improve the mechanical strength ofthe interconnection. Also, where the pad is longer than wide, thetolerance for misalignment of solder mask openings (or bump) isincreased; particularly where the pad is at the end of the trace, anelongated pad can reduce the likelihood that a solder mask opening (orbump) will be situated off the end of the pad.

The solder mask openings shown by way of example in FIGS. 4, 6, 8 and 9are generally round (circular or elliptical), but according to theinvention the solder mask opening may be shaped in any of a variety ofways. It may be useful for example, to provide a generally rectangularsolder mask opening, either square or elongated, as shown in FIGS. 16A,16B, (showing a generally rectangular pad as an example). A square orrectangle of a given width has a greater area than a circle or ellipsehaving the same width (diameter, short axis). For this reason a squareor rectangular mask opening has a capacity to hold a greater quantity ofsolder paste (or other fusible material), and accordingly this mayprovide an advantage where a fusible material such as a solder paste isto be applied to the mating surfaces on the narrow pads prior to matingwith the bumps (described in more detail below). Also, it may be easierto print a fusible material into a square or rectangular mask openingthan into a circular or elliptical mask opening, because there isgreater tolerance for misalignment in the printing process. Also, givena width limitation for the mask opening, a square or rectangular maskopening provides a greater open area for mounting a large bump on thepad during the interconnection process.

Various narrow pad configurations according to embodiments of theinvention are shown in FIG. 17 by way of example in relation to acircular mask opening 174 in a solder mask 176 . The mask opening ineach example has a width (diameter) Wm, which may be, for example, about90 um. A bump-on-lead configuration is shown at 173 . The lead or trace172 has a nominal (design) width WL, which may be, for example, about 30um. A narrow pad having a rectangular shape is shown at 175 . In thisexample the lead or trace at which the narrow pad is formed has anominal (design) width WL′, which may be, for example, about 30 um. Therectangular narrow pad has a width WP′, which may be, for example, about45 um. A narrow pad having an oval shape is shown at 177 formed at awider lead or trace, having a nominal (design) width WL″, which may be,for example, about 50 um. A narrow pad having a rectangular shapeexpanded with an oval shape is shown at 179 . In this example thenarrower lead or trace at which the narrow pad is formed has a nominal(design) width WL′″, which may be, for example, about 30 um. Therectangular portion of the narrow pad 179 has a width WP″, which may be,for example, about 45 um; and the oval expanded portion has a width WPE,which may be for example, about 50 um.

Various solder mask opening configurations according to embodiments ofthe invention are shown in FIG. 18 by way of example in relation to alead (or trace) or narrow pad 182. In these examples the lead or narrowpad at the interconnect site has a width WL, which may be, for example,about 40 um. In a first example, a circular solder mask opening 185having a width (diameter) Wm, which may be, for example, about 90 um,exposes an interconnect site portion 183 . In a second example arectangular solder mask opening 187 having a width (across the lead ornarrow pad) Wm′, which may be, for example, about 80 um, and a lengthLm′, which may be, for example, about 120 um., exposes an interconnectsite portion 183′. In a third example an elliptical solder mask opening189 having a width (across the lead or narrow pad) Wm″, which may be,for example, about 80 um, and a length Lm″, which may be, for example,about 120 um., exposes an interconnect site portion 183″. Both therectangular opening 187 and the oval opening 189 expose a greater length(hence, area) of the lead or pad at the site 183″, 183″ than does thecircular solder mask opening 185, even though the circular opening inthis example has a greater diameter. This provides a greater area forsolder reflow during the interconnect process, and can result in a morerobust interconnection. The area exposed by the rectangular opening 187is slightly greater than that provided by the elliptical opening 189having the same width and length; and moreover, the area would bereduced if there were a slight misalignment of the elliptical opening,but not by a slight misalignment of the rectangular opening. As apractical matter, however, an design rectangular opening may have moreor less rounded corners because of resolution limitations in processesfor patterning openings in the solder mask dielectric.

In some illustrative examples according to the invention, the diameterof the bump base on the die to be mounted may be about 90 um, and thenarrow interconnect pad is formed on the substrate to a width in a rangeabout 25 um (where the trace width is less than about 25 um) to about 50um. This provides a significant improvement in routing density, ascompared with a substrate having a conventional capture pad having amuch larger diameter, which may be typically two to four times as greatas the trace width.

As FIGS. 4 and 6 illustrate, bump-on-narrow-pad interconnect accordingto the invention can provide a significantly higher signal trace escaperouting density. Also, as FIGS. 4 and 6 illustrate, the BONPinterconnect according to this aspect of the invention does not requireuse of a solder mask to define the solder contour at the interconnectsite.

The BONP interconnection structure of embodiments such as are shown byway of example in FIGS. 4, 5, 6 and 7 can be produced according to theinvention by any of several methods, not requiring a solder mask. Ingeneral, interconnect bumps (typically solder bumps) are affixed ontointerconnect pads on the active side of the die. A die attach surface ofthe substrate (termed the “upper” surface) has an upper metal layerpatterned to provide the traces and narrow pads at interconnect sites asappropriate for interconnection with the arrangement of bumps on theparticular die. In a preferred method of the invention, an encapsulatingresin adhesive is employed to confine the solder flow during a meltphase of the interconnection process.

FIGS. 8 and 9 show two examples of a portion of a bump-on-narrow-padflip chip interconnection according to other embodiments of theinvention, in a diagrammatic sectional view taken in a plane parallel tothe substrate surface. Certain features are shown as if transparent.According to this aspect of the invention a solder mask is provided,which may have a nominal mask opening diameter in the range about 80 umto 90 um. Solder mask materials can be resolved at such pitches and,particularly, substrates can be made comparatively inexpensively withsolder masks having 90 um openings and having alignment tolerances plusor minus 25 um. In some embodiments laminate substrates (such as 4 metallayer laminates), made according to standard design rules, are used. Inthe embodiments of FIGS. 8 and 9, for example, the traces may be at ˜90um pitch and the narrow pads may be in a 270 um area array, providing aneffective escape pitch ˜90 um across the edge of the die footprint,indicated by the broken line 81.

In embodiments as in FIGS. 8 and 9 a no-flow underfill is not required;a conventional capillary underfill can be employed.

In embodiments as in FIG. 8 the interconnection is achieved by matingthe bumps directly onto an narrow interconnect pad 84 on a narrow leador trace 83 patterned on a dielectric layer on the die attach surface ofthe substrate 82; the solder mask 86 serves to limit flow of solderwithin the bounds of the mask openings 88, preventing solder flow awayfrom the interconnect site along the solder-wettable lead. The soldermask may additionally confine flow of molten solder between leads, orthis may be accomplished in the course of the assembly process.

In embodiments as in FIG. 9, as in FIG. 8, narrow pads on traces 93 arepatterned on a dielectric layer on the die attach surface of thesubstrate 92. Solder paste is provided at the interconnect sites (narrowpads) 94 on the leads 93, to provide a fusible medium for theinterconnect. The openings 98 in the solder mask 96 serve to define thepaste. The paste is dispensed, for example by a standard printingprocess, then is reflowed, and then may be coined if necessary toprovide uniform surfaces to meet the balls. The solder paste can beapplied in the course of assembly using a substrate as described abovewith reference to FIG. 8; or, a substrate may be provided with pastesuitably patterned prior to assembly. Other approaches to applyingsolder selectively to the interconnect sites may be employed in thesolder-on-narrow-pad embodiments of the invention, including electrolessplating and electroplating techniques. The solder-on-narrow-padconfiguration provides additional solder volume for the interconnect,and can accordingly provide higher product yield, and can also provide ahigher die standoff.

Accordingly, in some embodiments the solder-on-narrow-pad configurationaccording to the invention is employed for interconnection of a diehaving high-melting temperature solder bumps (such as a high-lead [highPb] solder, conventionally used for interconnection with ceramicsubstrates) onto an organic substrate. The solder paste can be selectedto have a melting temperature low enough that the organic substrate isnot damaged during reflow. To form the interconnect in such embodimentsthe high-melting interconnect bumps are contacted with thesolder-on-narrow-pad sites, and the remelt fuses thesolder-on-narrow-pad to the bumps. Where a noncollapsible bump is used,together with a solder-on-narrow-pad process, no preapplied adhesive isrequired, as the displacement or flow of the solder is limited by thefact that only a small quantity of solder is present at eachinterconnect, and the noncollapsible bump prevents collapse of theassembly.

In other embodiments the solder-on-narrow-pad configuration according tothe invention is employed for interconnection of a die having eutecticsolder bumps.

One embodiment of a preferred method for making a bump-on-narrow-padinterconnection is shown diagrammatically in FIGS. 10A-10C.

Referring to the FIGs., a substrate 112 is provided, having at least onedielectric layer and having a metal layer on a die attach surface 113,the metal layer being patterned to provide circuitry, particularlynarrow interconnection pads 114 on traces or leads, on the die attachsurface. The substrate 112 is supported, for example on a carrier orstage 116, with a substrate surface opposite the die attach surface 113facing the support. A quantity of an encapsulating resin 122 isdispensed over the die attach surface 113 of the substrate, covering atleast the narrow interconnection pads 114 on the leads. A die 102 isprovided, having bumps 104 attached to die pads (not shown in the FIG.)on the active side 103. The bumps include a fusible material whichcontacts the mating surfaces of the narrow pads. A pick-and-place tool108 including a chuck 106 picks up the die by contact of the chuck 106with the backside 101 of the die. Using the pick-and-place tool, the dieis positioned facing the substrate with the active side of the dietoward the die attach surface of the substrate, as shown in FIG. 10A;and the die and substrate are aligned and moved one toward the other(arrow M) so that the bumps 104 contact the corresponding narrowinterconnection pads 114 on the traces (leads) on the substrate. Then aforce is applied (arrow F) to press the bumps 105 onto the matingsurfaces 134 at the narrow pads 115 on the leads, as shown in FIG. 10B.The force must be sufficient at least to displace the adhesive 122 frombetween the bumps and the mating surfaces 134 at the narrowinterconnection pads 115. The bumps may be deformed by the force,breaking the oxide film on the contacting surface of the bumps and/or onthe mating surface of narrow pads. The deformation of the bumps mayresult in the fusible material of the bumps being pressed onto the topand over the edges of the narrow pads. The adhesive is caused to cure atleast partially, as shown at 132, as for example by heating to aselected temperature. At this stage the adhesive need only be partiallycured, that is, only to an extent sufficient subsequently to preventflow of molten solder along an interface between the adhesive and theconductive traces. Then the fusible material of the bumps 105 is meltedand then is re-solidified, forming a metallurgical interconnectionbetween the bump 105 and narrow pad 115, and the adhesive curing iscompleted, to complete the die mount and to secure the electricalinterconnection at the mating surface (now an interconnect interface)144, as shown generally at 140 in FIG. 10C. In the plane of thesectional view shown in FIG. 1C, interconnection is formed betweencertain of the bumps 145 and corresponding narrow interconnection pads155 on certain of the leads, as for example in a configuration as inFIG. 6. Other leads 156 are interconnected on narrow interconnectionpads at other localities, which would be visible in other sectionalviews. A comparatively high trace density is shown. The curing of theadhesive may be completed prior to, or concurrently with, or followingmelting the solder. Typically, the adhesive is a thermally curableadhesive, and the extent of curing at any phase in the process iscontrolled by regulating the temperature. The components can be heatedand cured by raising the temperature of the chuck on the pick and placetool, or by raising the temperature of the substrate support, forexample.

The process is shown in further detail in FIGS. 11A-11D. In FIG. 1A, asubstrate 212 is provided on a die attach surface with conductive(metal) traces, and narrow interconnection pads 214 at interconnectsites on the traces are covered with an adhesive 222. The die 202 ispositioned in relation to the substrate 212 such that the active side ofthe die faces the die attach side of the substrate, and is aligned(arrows A) such that bumps 204 on the die are aligned with correspondingmating surfaces on narrow pads 214. The die and the substrate are movedtoward one another so that the bumps contact the respective matingsurfaces on the narrow pads. Then as shown in FIG. 11B a force isapplied to move the bumps 205 and narrow pads 215 against one another,displacing the adhesive as shown at 232 in FIG. 1I B, and deforming thebumps onto the mating surfaces 234 and over the edges of the narrowpads. Deformation of the bumps on the narrow pads breaks the oxide filmon the contact surfaces of the bumps and the mating surfaces of thenarrow pads, establishing a good electrical connection, and deformationof the bumps over the edges of the narrow pads helps establish a goodtemporary mechanical connection. As in the example of FIG. 10A-10C, thenarrow interconnection pads of certain of the traces 216 are out of theplane of FIG. 11B. Heat is applied to partially cure the adhesive asshown at 236 in FIG. 11C. Then heat is applied to raise the temperatureof the bumps sufficiently to cause the fusible material of the bumps tomelt, as shown in FIG. 11D. This substantially (though not necessarilyfully) completes the cure of the adhesive 246 and completes themetallurgical interconnection of the bumps 245 onto the mating surfaces244 at the narrow interconnection pads 215. The cured adhesivestabilizes the die mount.

In an alternative embodiment of a preferred method, the adhesive can bepre-applied to the die surface, or at least to the bumps on the diesurface, rather than to the substrate. The adhesive can, for example, bepooled in a reservoir, and the active side of the die can be dipped inthe pool and removed, so that a quantity of the adhesive is carried onthe bumps; then, using a pick-and-place tool, the die is positionedfacing a supported substrate with the active side of the die toward thedie attach surface of the substrate, and the die and substrate arealigned and moved one toward the other so that the bumps contact thecorresponding traces (leads) on the substrate. Such a method isdescribed in U.S. Pat. No. 6,780,682, Aug. 24, 2004, which is herebyincorporated by reference. Then forcing, curing, and melting are carriedout as described above.

A force and temperature schedule for a process according to theinvention is shown diagrammatically by way of example in FIG. 12. Inthis chart, time runs from left to right on the horizontal axis; a forceprofile 310 is shown as a thick solid line, and a temperature profile320 is shown as a dotted line. The temperature profile begins at atemperature in the range about 80° C.-about 90° C. The force profilebegins at essentially zero force. Beginning at an initial time t_(i) theforce is rapidly (nearly instantaneously) raised 312 from F_(i) to adisplacement/deformation force F_(d) and held 314 at that force for atime, as discussed below. F_(d) is a force sufficiently great todisplace the adhesive away from between the bumps and the matingsurfaces of the narrow interconnection pads; and, preferably, sufficientto deform the fusible (narrow pad-contacting) portion of the bumps ontothe mating surface, breaking the oxide films and forming a goodmetal-to-metal (metallurgical) contact, and, in some embodiments, overthe edges of the narrow pads to establish a mechanical interlock of thebumps and the narrow pads (“creep” deformation). The total amount offorce required will depend upon the bump material and dimensions andupon the number of bumps, and can be determined without undueexperimentation. As the force is raised, the temperature is also rapidlyraised 322 from an initial temperature T_(i) to a gel temperature Tg.The gel temperature Tg is a temperature sufficient to partially cure theadhesive (to a “gel”). Preferably, the force and temperature ramps areset so that there is a short lag time t_(def), following the moment whenF_(d) is reached and before T_(g) is reached, at least long enough topermit the elevated force to displace the adhesive and to deform thebumps before the partial cure of the adhesive commences. The assembly isheld 314, 324 at the displacement/deformation pressure F_(d) and at thegel temperature T_(g) for a time t_(gel) sufficient to effect thepartial cure of the adhesive. The adhesive should become sufficientlyfirm that it can subsequently maintain a good bump profile during thesolder remelt phase—that is, sufficiently firm to prevent undesirabledisplacement of the molten fusible material of the bump, or flow of themolten fusible material along the narrow pads and leads. Once theadhesive has partially cured to a sufficient extent, the pressure may beramped down rapidly 318 to substantially no force (weight of thecomponents). The temperature is then rapidly raised further 323 to atemperature T_(m) sufficient to remelt the fusible portions (solder) ofthe bumps, and the assembly is held 325 at the remelt temperature T_(m)for a time t_(melt/cure) at least sufficient to fully form the solderremelt on the narrow pads, and preferably sufficient to substantially(though not necessarily fully) cure the adhesive. Then the temperatureis ramped down 328 to the initial temperature T_(i), and eventually toambient. The process outlined in FIG. 12 can run its course over a timeperiod of 5-10 seconds.

The adhesive in embodiments as in FIG. 12 may be referred to as a“no-flow underfill”. In some approaches to flip chip interconnection,the metallurgical interconnection is formed first, and then an underfillmaterial is flowed into the space between the die and the substrate. The“no-flow underfill” according to the invention is applied before the dieand the substrate are brought together, and the no-flow underfill isdisplaced by the approach of the bumps onto the narrow pads, and by theopposed surfaces of the die and the substrate. The adhesive for theno-flow underfill adhesive according to the invention is preferably afast-gelling adhesive—that is, a material that gels sufficiently at thegel temperature in a time period in the order of 1-2 seconds. Preferredmaterials for the no-flow underfill adhesive include, for example,so-called non-conductive pastes, such as those marketed by ToshibaChemicals and by Loktite-Henkel, for example.

Alternative bump structures may be employed in the bump-on-narrow-padinterconnects according to the invention. Particularly, for example,so-called composite solder bumps may be used. Composite solder bumpshave at least two bump portions, made of different bump materials,including one which is collapsible under reflow conditions, and onewhich is substantially non-collapsible under reflow conditions. Thenon-collapsible portion is attached to the interconnect site on the die;typical conventional materials for the non-collapsible portion includevarious solders having a high lead (Pd) content, for example; and gold(Au), for example. The collapsible portion is joined to thenon-collapsible portion, and it is the collapsible portion that makesthe connection with the narrow interconnect pad according to theinvention. Typical conventional materials for the collapsible portion ofthe composite bump include eutectic solders, for example.

An example of a bump-on-narrow-pad interconnect employing a compositebump is shown in a diagrammatic sectional view in FIG. 13. Referring nowto FIG. 13, die 302 is provided on die pads in the active side of thedie with composite bumps that include a noncollapsible portion 345 and acollapsible portion 347. The collapsible portion may be, for example, aeutectic solder or a relatively low temperature melt solder). Thecollapsible portion contacts the mating surface of the narrow pad and,where deformation of the fusible portion of the bump over the narrow padis desired, the collapsible portion of the bump is deformable under theconditions of force employed. The noncollapsible portion may be, forexample, a solder having a high lead (Pb) content. The noncollapsibleportion does not deform when the die is moved under pressure against thesubstrate during processing, and does not melt during the reflow phaseof the process. Accordingly the noncollapsible portion can bedimensioned to provide a standoff distance between the active surface ofthe die and the die attach surface of the substrate.

As may be appreciated, the bumps in embodiments as shown in, forexample, FIGS. 4, 5, 6 and 7 need not necessarily be fully collapsiblebumps. The structures shown in those FIGs. may alternatively be madeusing composite bumps, or using non-collapsible bumps (high-Pb, or Au)in a solder-on-narrow-pad method, as described above.

And, as may be appreciated in view of the foregoing, an interconnect asappears for example in FIG. 13 can be formed by bringing a non-compositenon-collapsible bump (high-Pb, Au) into contact with a narrowinterconnect pad provided on the mating surface with a fusible material(such as, for example, a eutectic solder or a relatively low temperaturemelt solder, which may be provided as a solder paste). Or, the narrowinterconnect pad may be provided on the mating surface with a fusiblematerial and the bumps may be composite bumps, also provided with acollapsible (fusible) portion. Where the narrow interconnect pads areprovided on the mating surface with a fusible material, it may bepreferred to employ a solder mask, followed by a capillary underfill, inthe process.

Other embodiments are within the following claims.

1. A flip chip interconnection, comprising a solder bump attached to aninterconnect pad on a die and mated onto a pad at an interconnect siteon a substrate, wherein a width of the pad at the interconnect site isless than a nominal width of the contact interface between the bump andthe die pad.
 2. The flip chip interconnection of claim 1 wherein thewidth of the pad at the interconnect site is at least about 20% as greatas the nominal width of the contact interface between the bump and thedie pad.
 3. The flip chip interconnection of claim 1 wherein the widthof the pad at the interconnect site is at least about 25% as great asthe nominal width of the contact interface between the bump and the diepad.
 4. The flip chip interconnection of claim 2 wherein the width ofthe pad at the interconnect site is less than about 80% as great as thenominal width of the contact interface between the bump and the die pad.5. The flip chip interconnection of claim 2 wherein the width of the padat the interconnect site is less than about 60% as great as the nominalwidth of the contact interface between the bump and the die pad.
 6. Theflip chip interconnection of claim 1 wherein the width of the pad at theinterconnect site is greater than about 120% of a trace design rulewidth.
 7. A flip chip interconnection, comprising a solder bump attachedto an interconnect pad on a die and mated onto a pad at an interconnectsite on a substrate, wherein a width of the pad at the interconnect siteis less than a design width of the contact interface between the bumpand the die pad.
 8. The flip chip interconnection of claim 7 wherein thewidth of the pad at the interconnect site is at least about 20% as greatas the design width of the contact interface between the bump and thedie pad.
 9. The flip chip interconnection of claim 7 wherein the widthof the pad at the interconnect site is at least about 25% as great asthe design width of the contact interface between the bump and the diepad.
 10. The flip chip interconnection of claim 8 wherein the width ofthe pad at the interconnect site is less than about 80% as great as thedesign width of the contact interface between the bump and the die pad.11. The flip chip interconnection of claim 8 wherein the width of thepad at the interconnect site is less than about 60% as great as thedesign width of the contact interface between the bump and the die pad.12. The flip chip interconnection of claim 1 wherein the width of thepad at the interconnect site is greater than about 120% of a nominaltrace width.
 13. A flip chip semiconductor package comprising aplurality of solder bumps each attached to an interconnect pad on a dieand mated onto a corresponding a pad at an interconnect site on asubstrate, wherein a width of the pad at the interconnect site is lessthan a nominal width of the contact interface between the bump and thedie pad.
 14. The flip chip semiconductor package of claim 13 wherein thewidth of the pad at the interconnect site is at least about 20% as greatas the nominal width of the contact interface between the bump and thedie pad.
 15. The flip chip semiconductor package of claim 13 wherein thewidth of the pad at the interconnect site is at least about 25% as greatas the nominal width of the contact interface between the bump and thedie pad.
 16. The flip chip semiconductor package of claim 14 wherein thewidth of the pad at the interconnect site is less than about 80% asgreat as the nominal width of the contact interface between the bump andthe die pad.
 17. The flip chip semiconductor package of claim 14 whereinthe width of the pad at the interconnect site is less than about 60% asgreat as the nominal width of the contact interface between the bump andthe die pad.
 18. The flip chip semiconductor package of claim 14 whereinthe width of the pad at the interconnect site is greater than about 120%of a nominal trace width.
 19. A flip chip semiconductor packagecomprising a plurality of solder bumps each attached to an interconnectpad on a die and mated onto a corresponding a pad at an interconnectsite on a substrate, wherein a width of the pad at the interconnect siteis less than a design width of the contact interface between the bumpand the die pad.
 20. The flip chip semiconductor package of claim 19wherein the width of the pad at the interconnect site is at least about20% as great as the design width of the contact interface between thebump and the die pad.
 21. The flip chip semiconductor package of claim19 wherein the width of the pad at the interconnect site is at leastabout 25% as great as the design width of the contact interface betweenthe bump and the die pad.
 22. The flip chip semiconductor package ofclaim 20 wherein the width of the pad at the interconnect site is lessthan about 80% as great as the design width of the contact interfacebetween the bump and the die pad.
 23. The flip chip semiconductorpackage of claim 20 wherein the width of the pad at the interconnectsite is less than about 60% as great as the design width of the contactinterface between the bump and the die pad.
 24. The flip chipsemiconductor package of claim 20 wherein the width of the pad at theinterconnect site is greater than about 120% of a nominal trace width.25. The flip chip interconnection of claim 1, the interconnection beingnon-solder mask defined.
 26. The flip chip interconnection of claim 1,the interconnection being solder mask defined.
 27. A substrate for flipchip interconnection of a die, the die having a bumps each attached on adie pad, the substrate comprising a patterned metal layer having pads atinterconnect sites, wherein a width of a said pad at a said interconnectsite is less than a nominal width of a contact interface between a saidbump and the die pad.
 28. The substrate of claim 27 wherein the width ofthe pad at the interconnect site is at least about 20% as great as thenominal width of the contact interface between the bump and the die pad.29. The substrate of claim 27 wherein the width of the pad at theinterconnect site is at least about 25% as great as the nominal width ofthe contact interface between the bump and the die pad.
 30. Thesubstrate of claim 28 wherein the width of the pad at the interconnectsite is less than about 80% as great as the nominal width of the contactinterface between the bump and the die pad.
 31. The substrate of claim28 wherein the width of the pad at the interconnect site is less thanabout 60% as great as the nominal width of the contact interface betweenthe bump and the die pad.
 32. The substrate of claim 28 wherein thewidth of the pad at the interconnect site is greater than about 120% ofa trace design rule width.
 33. A method for forming a flip chipinterconnection, comprising providing a substrate as recited in claim27; supporting the substrate and the die; dispensing a quantity of acurable adhesive on the substrate; positioning the die with the activeside of the die toward the die attach surface of the substrate, andaligning the die and substrate and moving one toward the other so thatthe bumps contact the corresponding narrow interconnection pads on thesubstrate; applying a force to press the bumps onto the mating narrowpads, sufficient to displace the adhesive from between the bump and themating narrow pad; at least partially curing the adhesive; melting andthen re-solidifying the solder, forming a metallurgical interconnectionbetween the bump and the pad.
 34. The method of claim 33 whereindispensing a quantity of a curable adhesive on the substrate covers atleast the interconnection pads on the substrate.